1. Field of the Invention
The present invention relates to a bidirectional optical module and more specifically, it relates to a bidirectional optical module to be included in an OTDR used in applications such as measurement of a fracture in an optical fiber communication network.
2. Description of the Related Art
A measuring device such as an optical fiber sensor, which executes measurement by using light in an optical communication system or the like, includes a light source that emits light and a light receiving unit that receives the light. A measuring device utilized in maintenance, management and the like of an optical communication system includes a light source that emits measurement light to be used for purposes of measurement to a measurement target optical fiber and a light receiving unit that receives light transmitted through the measurement target optical fiber.
For instance, an OTDR (Optical Time Domain Reflectometer) may be utilized in the installation, maintenance and the like of an optical fiber in order to monitor the state of the optical fiber through which light signals are transmitted for data communication in an optical communication system. An OTDR executes measurement to determine the conditions of the measurement target optical fiber, e.g., whether or not a disconnection has occurred at the measurement target optical fiber, the extent of loss or the like, by repeatedly inputting pulse light to the measurement target optical fiber and measuring the level of light reflected from the measurement target optical fiber, the level of light scattered to the rear and the length of time over which the light is received.
An OTDR includes a bidirectional optical module, a BIDI (bidirectional) module or the like, having a transmission unit and a reception unit housed in a single case. The advent of the FTTH (fiber to the home) technologies in recent years has resulted in these modules being offered at more affordable prices and thus, they have come to be used in a wide range of applications including other types of measuring devices and optical communication systems as well as OTDRs.
For example, an OTDR equipped with a bidirectional optical module includes, as shown in FIG. 4, a bidirectional optical module 1, an LD drive unit 2, a sampling unit 3, a signal processing unit 4, and a display unit 5.
The bidirectional optical module 1 outputs pulse light to a measurement target optical fiber 7 via a measurement connector 6 and receives the light returning from the measurement target optical fiber 7. The LD drive unit 2 drives a light source disposed within the bidirectional optical module 1. The sampling unit 3 is a functional unit that converts an electrical signal (photocurrent) from a light receiving unit within the bidirectional optical module 1 to a voltage and samples the voltage resulting from the conversion. The signal processing unit 4 is a functional unit that engages the bidirectional optical module 1 to output pulse light via the LD drive unit 2 and engages the sampling unit 3 in a sampling operation. In addition, the signal processing unit 4 executes arithmetic operation processing on the electrical signal sampled by the sampling unit 3. The display unit 5 is a functional unit that indicates the signal processing results and may be constituted with, for instance, a display device.
The conventional bidirectional optical module 1 includes, for example, as shown in FIG. 5, optical separators 10a and 10b, multiplexing/demultiplexing couplers 20 and 30, a lens 40, and a light receiving element 50 (for example, Japanese Patent Application Laid-Open No. 2004-145136).
The optical separator 10a includes, for example, as shown in FIG. 6, a semiconductor laser 11, a non-reciprocal unit 13, optical fibers 15 and 18, a refractive prism 16, and lenses 12, 14, and 17. Light emitted from the semiconductor laser 11 of the optical separator 10a is made parallel by the lens 12 and then, the light having a polarization plane in a predetermined direction with respect to an incidence plane when entering the non-reciprocal unit 13 enters the non-reciprocal unit 13 at a predetermined angle θ. After entering the non-reciprocal unit 13, the light passes through the non-reciprocal unit 13 before being coupled to the optical fiber 15 connected to the multiplexing/demultiplexing coupler 20 shown in FIG. 5 via the lens 14. The optical separator 10b is configured in the same manner as the optical separator 10a. 
The optical separators 10a and 10b emit lights of mutually different wavelengths λ1 and λ2 from the semiconductor laser 11. Lights of these wavelengths λ1 and λ2 enter the multiplexing/demultiplexing coupler 20 via the optical fiber 15 of the optical separators 10a and 10b. The light multiplexed by the multiplexing/demultiplexing coupler 20 is output to the measurement target optical fiber 7 via the measurement connector 6 before being reflected by a fracture (or a connecting point) in the measurement target optical fiber 7. The reflected light enters the multiplexing/demultiplexing coupler 20 via the measurement connector 6 as a returning light. The multiplexing/demultiplexing coupler 20 demultiplexes the returning light from the measurement target optical fiber 7 before a light of the wavelength λ1 being output to the optical separator 10a and a light of the wavelength λ2 to the optical separator 10b. 
The demultiplexed returning light of the wavelength λ1 passes through the non-reciprocal part 13 via the lens 14 in the optical separator 10a before entering the refractive prism 16. After entering the refractive prism 16, the light travels through the refractive prism 16 while being refracted before being coupled to the optical fiber 18 connected to the multiplexing/demultiplexing coupler 30 via the lens 17. Similarly, the returning light of the wavelength λ2 demultiplexed by the multiplexing/demultiplexing coupler 20 passes through the optical separator 10b before being coupled to the optical fiber 18 connected to the multiplexing/demultiplexing coupler 30. Then, the lights of the wavelengths λ1 and λ2 incident on the multiplexing/demultiplexing coupler 30 from the optical fiber 18 of the optical separators 10a and 10b are multiplexed by the multiplexing/demultiplexing coupler 30 and then condensed before being coupled to the light receiving element 50.
By using the OTDR described above, the position of a fracture of the measurement target optical fiber 7 can be detected based on a time between when an emitted pulsed light is generated in the semiconductor laser 11 and when a returning light reflected by a fracture reaches the light receiving element 50. The bidirectional optical module 1 for OTDR configured as shown in FIG. 5 has a circulator function and can separate light with a low loss, providing high optical power and high sensitivity.
Patent reference literature 1: Japanese Laid Open Patent Publication No. 2004-145136